1. Field of the Invention
The present invention relates to a method of manufacturing a polyurethane foam and, more particularly, to a method of manufacturing a polyurethane foam having uniform cells.
2. Description of the Prior Art
One- and two-stage methods are available in a conventional method of manufacturing a polyurethane foam. The one-stage method called a one-shot method has been frequently employed due to productivity and any other reasons. A schematic arrangement for practicing the conventional one-shot method is shown in FIG. 1. Referring to FIG. 1, reference numeral 1 denotes a first material tank which stores a polypropylene glycol (PPG) 3 as one major material; and 2, a second material tank which stores an organic isocyanate (e.g., tolylenediisocyanate (TDI)) 4 as the other major material. Material supply pipes 5 and 6 are respectively connected to the first and second material tanks 1 and 2 to supply the PPG 3 and the TDI 4 to a mixer 7. Valves 8 and 9 are arranged along the material supply pipes 5 and 6, respectively. Pumps 10 and 11 are also arranged midway along the material supply pipes 5 and 6, respectively. Upon driving of the pumps 10 and 11, the PPG 3 and TDI 4 can be supplied to the mixer 7. The amount of materials to be supplied to the mixer 7 can be suitably adjusted. Silicone, a catalyst, and other materials are also supplied to the mixer 7. The materials supplied to the mixer 7 are mixed to prepare a foamable solution. This solution is delivered from a mixing head arranged in the mixer 7.
A polyurethane foam can be manufactured by using the above apparatus as follows. The valves 8 and 9 are controlled to open to supply the PPG and the TDI from the material tanks 1 and 2 to the mixer 7. At the same time, a catalyst, a foam stabilizer (silicone), water, a low-boiling material, and the like are also supplied from a separate system to the mixer 7. These materials are sufficiently mixed in the mixer 7, and a foamable solution is injected from an injection port to molds. Foaming of the foamable solution injected into the molds is started immediately after the injection, and the foamable solution is gelled to form a polyurethane foam.
In the conventional method of manufacturing a polyurethane foam by using the above apparatus, the materials cannot always be satisfactorily mixed by the mixer, and various problems caused by insufficient mixing are posed. In the conventional method, the PPG, the TDI, water, the gelling catalyst, the foaming catalyst, the foam stabilizer, and other materials are supplied to the mixer, and stirring, mixing, and injection are continuously performed by the mixer, thus causing unsatisfactory mixing. When stirring time is increased to sufficiently mix the materials by the mixer, reactions of the materials are started to increase a viscosity of a foamable solution. An increase in stirring time is therefore impractical. For this reason, efficient stirring within a short period of time must be performed. The structure of the mixer has been improved to perform efficient stirring. However, the improvements cannot always be satisfactory, and many problems are left unsolved. According to the conventional method, the chemical reactions during foaming are often unbalanced to cause cracking or abnormal reactions which cause scorching in the resultant foams. As a result, low-density, high-hardness foams cannot be obtained by the conventional method.
A prepolymer method is also available as a conventional method of manufacturing an urethane foam. According to this method, since the reaction between the PPG and the TDI is completed, the reaction product has a high viscosity. The reaction product cannot be easily reacted with a catalyst, water, and the like. For this reason, a TDI must be added to the reaction product to control the chemical reaction, resulting in low productivity and high cost.